Technology of this type may be seen in U.S. Pat. No. 7,460,681, EP2375784, US2011/044485, WO2014090282 and U.S. Pat. No. 8,798,299.
In general, sound generators, such as receivers for hearing aids or hearables, are sources of electromagnetic radiation due to the AC current in the coil and displacement of the ferromagnetic armature in the airgap between two permanent magnets. Such a magnetic field may be picked up by nearby elements, such as a telecoil also placed in the hearing aid with consequent stability issues and feedback.
Especially in BTE and ITE applications, a telecoil will be positioned so close to the receiver that magnetic shielding made up of mu-metal is desired. In this type of design it has been found that the sound outlet and in some circumstances also the holes for the lead wires may be a key point, as the diameter of such holes can decrease the shielding performance due to the magnetic leak. A solution to this could be the increase of the shielding thickness but this will increase the size of the product.